FIG. 1 is a diagram of a typical prior art charged particle beam device 1, such as an electron microscope, which is described in Engelen et al., U.S. Pat. No. 5,280,178. The charged particle beam device 1 includes a column 3 capable of being evacuated to, for example, 10−7 Torr. An electron source 5 emits an electron beam along an optical axis 7, which beam is imaged on a specimen 11 by way of a dual condensor system 8 and an objective lens 9. The lower portion of objective lens 9, together with a diffraction lens, an intermediate lens and two projector lenses, all diagrammatically represented by the lens 10, form an image of the specimen 11 on a target 20 with a magnification of, for example 106.
In a microscope system such as charged particle beam device 1, the specimen 11 is typically supported by a specimen holder 13. The specimen holder 13 is connected to the column 3 through spherical bearing 17 so that specimen holder 13 can be displaced relative to the optical axis 7 over a distance of approximately 1 mm in a direction parallel to the optical axis 7 and also over a distance of approximately 1 mm in a direction which is normal to the optical axis 7. In addition, the specimen 11 may tilted by rotation of the specimen holder 13 about its axis. Tilting of the specimen 11 is important in order to obtain a number of images of the specimen 11 with different orientations at different angles. Such multi-angle imaging may be used for reconstruction of a three dimensional image of the specimen 11, for examination of diffraction images of the specimen 11, or, in the case where material analysis of the specimen 11 is being conducted, for optimizing the signal from an X-ray detector which can be arranged adjacent the upper poleshoe 9a of the objective lens 9 and which detects the X-rays generated in the specimen 11 by the electron beam.
According to the Rayleigh theory, the dimension of the smallest observable details in the specimen 11 is proportional to λ/a, where a is the numerical aperture of the objective lens 9 and λ is the wavelength of the electrons. For an electron energy amounting to 300 kV, the wavelength amounts to approximately 2×10−3 nm. Because of the spherical aberration caused by the objective lens 9, such a resolution cannot be achieved because a point situated on the optical axis 7 is imaged by the objective lens 9 as a spot having a diameter proportional to Csa3. Therein, Cs is the spherical aberration coefficient. A minimum resolution is obtained when the numerical aperture a is chosen between an as low as possible value for reducing the spherical aberration and an as high as possible value for maximizing the resolution according to Rayleigh. The dimension of the smallest observable details is then 0.067 Cs¼λ1/4. For high-resolution electron microscopes, a resolution of 0.17-0.20 nm can be achieved for a spherical aberration coefficient value of 1 mm. Such a low aberration coefficient can be achieved by making the distance between the poleshoes 9a and 9b equal to 1 mm. Because of the small poleshoe distance required for a high-resolution, the part of the specimen holder 13 to be introduced between the poleshoes must be very thin. Moreover, the specimen holder 13 must be sufficiently rigid to prevent image-disturbing vibrations of the specimen 11. It must also be possible to attach a specimen 11 which comprises, for example, a carbon film having a thickness of 20 nm, supported on a circular metal mesh commonly known as a grid, to the specimen holder 13 in a simple manner without causing damage to the specimen 11.
FIG. 2 is an isometric diagram of prior art specimen retaining device 21 that may form part of specimen holder 13 for holding specimen 11 in charged particle beam device 1. The specimen retaining device 21 is described in U.S. Pat. No. 5,280,178, and includes a supporting face 23 against which the specimen 11 may rest. The specimen 11 may be clamped against supporting face 23 by resilient retaining element 25, which comprises a contact portion in the form of an annular central portion 27 and three arms 29a, 29b and 29c. Each arm 29 includes a lug 31 which can resiliently engage a circumferential edge 33 of specimen retaining device 21. One of the lugs 31 is provided with detachment portion 35 for facilitating detachment of retaining element 25. Supporting face 23 is recessed relative to upper surface 39 of specimen retaining device 21 such that retaining element 25 bears against upright positioning edge 41 of specimen retaining device 21. A circular specimen 11 may thus be located on the plane of the supporting face 23 by the vertical edge 41, which comprises several sections of a cylindrical surface, and will be held in place by retaining element 25.
FIG. 3 is an isometric diagram of an alternate specimen retaining device 21 described in U.S. Pat. No. 5,280,178. A specimen 11 in this embodiment is clamped against the supporting face 23 by resilient tongues 43 and 45 with a resilient force, providing the well-known advantages of resilient clamping.
Specimen retaining device 21 shown in FIG. 2 has some tolerance to specimens of varying heights, and also has a rather low profile. However, it has proven difficult to use by some operators because of its requirement to align three points of engagement simultaneously between the clip 27 and the body 39. Further, it imposes a finite limit to the tilt-angles at which a specimen may be observed, since it presents a raised structure above the specimen around the entire perimeter of the specimen 11, and it requires a supporting structure below the specimen around the entirety of its perimeter. In the device 21 shown in FIG. 3, because the resilient tongues 43 and 45 are positioned at discrete locations around the perimeter of the specimen, they can be arranged so as not to interfere with viewing the specimen at high tilt angles when tilted about at least one axis. However, this approach suffers from the disadvantage that the specimen must be slid underneath the clips simultaneously to experience a restraining force from the clips. This situation makes loading difficult, and can easily damage a fragile specimen.
A further alternative device for retaining and supporting a specimen is taught in Yanaka et al., U.S. Pat. No. 4,596,934. In this device, a specimen to be examined is placed on a circular specimen grid, and the grid is positioned within a cylindrical counterbore formed within a moveable specimen holder that may form a part of or be utilized in the manner of specimen holder 13 described in connection with FIG. 1. A C-shaped retaining spring is described which snap-fits into a corresponding internal groove formed within the specimen holder. This mechanism has limited ability to restrain specimens of varying thickness, and the C-shaped retaining spring has been found to be difficult to handle by some operators.
Another well known prior art method and device for restraining a specimen utilizes an externally-threaded ring which mates with internal threads formed in the body of the specimen holder. This mechanism can easily damage fragile specimens due to the rigid nature of the screw threads. Further, some applications require observing the specimen at high angles from normal (i.e., nearly parallel to its surface), for instance in order to re-construct a three-dimensional model using observations at multiple angles. The threaded ring presents a high profile normal to the plane of specimen support such that it obscures the view of the specimen at high angles and constrains observation to a limited range of angles. That is, the field of view on the specimen is a circle at a normal beam incidence, and is gradually reduced or “shadowed” into a cat's-eye shape by the threaded ring as the holder is tilted, until the field of view becomes negligible at a modest tilt angle, e.g., +/−50 degrees from normal.
Yet another prior art device for restraining a specimen is shown in FIG. 4. This device uses levers that can pivot about axes oriented parallel to the plane of specimen support in such a way that one end of each lever can contact the top surface of the specimen and bring a substantially normal restraining force to bear on it. Specifically, referring to FIG. 4, a supporting face 100 is the primary locating surface for the specimen. Jaws 103 and 104, forming a part of the levers, can be rotated around hinges 105 and 106 so as to press against the specimen with a force substantially normal to face 100. The force is applied and controlled by way of wire legs 101 and 102, which are made of a resilient material, to provide the known advantages of resilient clamping. For high-tilt viewing of the specimen, this geometry has the advantage that the levers comprising jaws 103 and 104 and wire legs 101 and 102 can occupy discrete positions around the perimeter of the specimen, and can be arranged so as not to interfere with viewing of the specimen at glancing angles when tilted about at least one axis. That is, the specimen may be tilted about at least one axis without any line-of-sight obstruction of a central area by the restraining mechanism. A commercial example of such a device, the model 670 Ultra High Tilt Holder sold by Gatan, Inc., is advertised to have the ability to tilt to +/−80 degrees from normal incidence without obscuring the specimen. However, the same pivoting motion that applies the restraining force is also used to retract the tips of the levers, i.e., jaws 103 and 104, away from the specimen area during loading and un-loading. Therefore, it is difficult for an operator to pause and inspect the positioning of the specimen and the levers prior to applying the restraining force. Also, in order to lift the jaw 103 and 104 of each lever away from the specimen, the wire legs 101 and 102 of each lever must pivot below the plane of specimen support, making it difficult for an operator to manipulate with precision. Adding to this difficulty is the fact that the operator is required to manipulate the wire legs 101 and 102 from the side opposite that from which the specimen is installed. Further, this device is difficult to make small, so as to fit easily in narrow spaces such as typically encountered in charged-particle instruments such as electron microscopes.
Another known form of specimen retention consists of two planar supporting surfaces, located substantially in a single plane, with means such as screws and washers for affixing each of two opposite ends of a specimen (usually in the form of a ribbon) to the two aforementioned planar surfaces. This is typically a feature of so called straining holders, in which one of the surfaces is movable so as to stretch the specimen while under observation. Known straining holders require the clamping mechanism to be removed entirely for the purpose of loading a specimen, and so are cumbersome to use.
All of these prior art specimen restraining devices use a stationary supporting face located below the specimen, and a moveable or removable restraining element located above the specimen. It is appreciated that the ability to view the specimen at high angles of tilt is affected both by the shape and dimensions of the restraining element(s) and the shape and dimensions of the supporting structure below the specimen. Hence, it is desired to make each of these elements as thin as practical to achieve viewing at high tilt, as the thickness of each will limit the practical viewing angle. However, it is also understood that the design and physical characteristics of the restraining element impose restrictions on the design and physical characteristics of the supporting structure, so that these elements are not independent of one another. For example, a threaded-ring restraining element requires mating threads in the counterbore, and exerts downward force via the specimen onto the supporting surface. The supporting structure must therefore be sufficiently large and strong to resist said force without permanent deformation. Likewise, a resilient supporting mechanism produces forces within the structure of the holder, which forces must be borne by the holder without permanent distortion.